代码随想录第三天 || 203.移除链表元素 || 707.设计链表 || 206.反转链表

203. 移除链表元素

文章：代码随想录 (programmercarl.com)

视频：https://www.bilibili.com/video/BV18B4y1s7R9

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        //删除头结点
        while (head != NULL && head->val == val)
        {
            ListNode* temp = head;
            head = head->next;
            delete temp;
        }

        //删除非头结点
        ListNode* cur = head;
        while (cur != NULL && cur->next != NULL)
        {
            if (cur->next->val == val)
            {
                ListNode* temp = cur->next;
                cur->next = cur->next->next;
                delete temp;
            }
            else {
                cur = cur->next;
            }
        }
        return head;
    }
};

707. 设计链表

文章：代码随想录 (programmercarl.com)

视频：https://www.bilibili.com/video/BV1FU4y1X7WD

class MyLinkedList {
public:
    struct ListNode {
        int val;
        ListNode* next;
        ListNode() : val(0), next(nullptr) {}
        ListNode(int x) : val(x), next(nullptr) {}
    };

    ListNode* _dummyHead;
    int _size;

    MyLinkedList() {
        _dummyHead = new ListNode(0);
        _size = 0;
    }
    
    int get(int index) {
        if (index > (_size - 1) || index < 0) {
            return -1;
        }
        ListNode* cur = _dummyHead->next;
        while (index-- != 0)
        {
            cur = cur->next;
        }
        return cur->val;
    }
    
    void addAtHead(int val) {
        ListNode* newNode = new ListNode(val);
        newNode->next = _dummyHead->next;
        _dummyHead->next = newNode;
        _size++;
    }
    
    void addAtTail(int val) {
        ListNode* cur = _dummyHead;
        ListNode* newNode = new ListNode(val);
        while (cur->next != NULL)
        {
            cur = cur->next;
        }
        cur->next = newNode;
        _size++;
    }
    
    void addAtIndex(int index, int val) {
        if (index > _size)
        {
            return;
        } 
        if (index < 0)
        {
            index = 0;
        }
        ListNode* cur = _dummyHead;
        while (index-- != 0)
        {
            cur = cur->next;
        }
        ListNode* newNode = new ListNode(val);
        newNode->next = cur->next;
        cur->next = newNode;
        _size++;
    }
    
    void deleteAtIndex(int index) {
        if (index >= _size || index < 0) {
            return;
        }
        ListNode* cur = _dummyHead;
        while (index--)
        {
            cur = cur->next;
        }
        ListNode* temp = cur->next;
        cur->next = cur->next->next;
        delete temp;
        _size--;
    }
};

206. 反转链表

文章：代码随想录 (programmercarl.com)

视频：https://www.bilibili.com/video/BV1nB4y1i7eL

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* pre = NULL;
        ListNode* cur = head;
        ListNode* temp;
        while (cur != NULL)
        {
            temp = cur->next;

            cur->next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
};

//对每一步的分析：
pre = null; cur = 1; temp = cur->next = 2; cur->next = pre = null; 1->null; pre = cur = 1; cur = temp = 2;
pre = 1; cur = 2; temp = cur->next = 3; cur->next = pre = 1; 2->1;pre = cur = 2; cur = temp = 3;
pre = 2; cur = 3; temp = cur->next = 4; cur->next = pre = 2;3->2;pre = cur = 3; cur = temp = 4;
pre = 3; cur = 4; temp = cur->next = 5; cur->next = pre = 3;4->3;pre = cur = 4; cur = temp = 5;
pre = 4; cur = 5; temp = cur->next = null; cur->next = pre = 4; 5->4; pre = cur = 5; cur = temp = null;
return pre;

打卡第三天，今天的练习对链表的细节更熟悉了，开始用MarkDown记笔记，反转链表的思路很精彩，学到了很多，对移除链表中的元素的代码从思路清晰到代码写懂，今天的学习很棒！